To enhance the performance of thin-film transistors (TFTs) and to reduce the temperature used in and the cost of the processes for producing TFTs, various materials have been studied for use as materials for forming channel layers of TFTs. In particular, materials that are prominently used for forming such channel layers are amorphous silicon, polycrystalline silicon, microcrystalline silicon, organic semiconductors, and the like.
In recent years, oxide semiconductors represented by amorphous In—Ga—Zn—O oxide semiconductors have been attracting attention as novel and promising materials for forming such channel layers. Since such oxide semiconductors have excellent semiconductor characteristics and can be formed at low temperatures in a large area, application of the oxide semiconductors to TFTs for backplanes of organic EL displays and liquid crystal displays has been studied. Such oxide semiconductors are mostly n-type semiconductors and only a few p-type oxide semiconductors are known. Such a few p-type oxide semiconductors function as p-type semiconductors in pn junction devices, however, few p-type oxide semiconductors function as p-channel TFTs. Recently, it has been reported that an epitaxial SnO film has good p-type semiconductor characteristics and functions as a p-channel TFT in Non Patent Literature 1.
Since there are no oxide semiconductors usable as p-channel TFTs, application of oxide semiconductors to devices is restricted to TFTs for backplanes, which can be constituted by TFTs having either n-type conduction or p-type conduction, and there are few applications of oxide semiconductors to logic circuits and the like that require complementary operations.
At present, a SnO film is a rare material that has p-type semiconductor characteristics and functions as a TFT. However, when application of such a SnO film to semiconductor devices such as TFTs is attempted, it is difficult to provide devices having a large area with an epitaxial SnO film on a single crystal substrate and a considerable increase in production cost is also expected. SnO has a thermodynamic metastable phase and hence it is difficult to provide a single-phase SnO film. Accordingly, when a polycrystalline SnO film is formed, the resultant film has a metal Sn phase, a SnO2 phase, or a mixed phase including a metal Sn phase and a SnO2 phase, which results in poor performance as a p-type semiconductor. It is reported in Patent Literature 1 that a single-phase polycrystalline SnO film is obtained by thermal decomposition spraying in which a SnF2 solution is used as a material. However, films obtained by this method generally have large surface irregularities and particles tend to be generated in the formation processes of such films. These disadvantages can cause problems such as degradation of performance of semiconductor devices or an increase in the level of defectiveness in the production processes of semiconductor devices.
Production of a complementary semiconductor device, which is a type of device including oxide semiconductors, requires individual formation of an n-type semiconductor film and a p-type semiconductor film. Accordingly, since the number of steps for forming films and accompanying steps is increased, an increase in production cost is expected.